1. Field of the Invention
The present invention relates, in general, to a method for fabricating GaN substrates and, more particularly, to the minimization of defects in GaN substrates, along with the method.
2. Description of the Prior Art
Recently, blue light elements utilizing GaN, such as light emitting diodes (LEDs) and laser diodes (LDs), have been one subject of great interest for many researchers and are now in a mature commercialization phase. As a consequence, such light elements find numerous applications in large size, color flat panel displays, signal lamps, interior lamps, high density light sources, high definition printing systems, optical communication, etc.
The substrates useful in the blue light elements are made of bulk GaN which is obtained by hydride vapor phase epitaxy. This technique is evaluated as being useful in minimizing the defects caused by the thermal expansion difference and lattice mismatch between substrates and thin films deposited thereon.
In order to produce thin films of high quality, these substrates should provide suitable surfaces. However, there remains a need for an improved polishing or pretreatment process that is adapted to accomplish sufficiently flat surfaces.
Using a diamond slurry, plate, or paste, alone, for example, a polishing process is conventionally conducted on bulk GaN substrates. The polished surfaces of the GaN substrates, however, do not show desired surface roughness because there exists a great difference in hardness between the diamond used and the bulk GaN. With expectations to solve this problem, sufficiently small sizes of diamond particles were used to polish GaN substrates, but could not yet bring about sufficiently reduced surface roughness in the GaN substrates.
The resulting surface roughness leads subsequent thin films to three-dimensional growth, making it impossible to obtain thin films of high quality. Upon the mechanical polishing with diamond particles, in addition, there are formed damaged layers whose defects are, in their entirety, transferred to subsequent thin films so that the thin films of high quality cannot be achieved which are as low in defect density as desirable. Consequently, devices of high properties cannot be fabricated by such conventional techniques.